The present technology refers to a method for detecting the presence of micro-holes and cracks in caps (or containers) at high speeds surpassing thousands caps per minute and an apparatus for enacting the aforementioned method, as described in their respective independent claims.
Spark inspection of injection molded bottle caps is known to be an effective way of controlling the quality of the caps by ensuring that they are free of micro tears or micro holes that can result from the molding process, particularly at high speed. One electrode is inserted into an inside of the cap while another is positioned on an outside of the cap, and high voltage is applied to the electrodes. If a spark is able to pass through the plastic, insulating cap, then there is a micro hole or tear in the cap. Such defects detected by spark testing are not efficiently detected by optical testing or by pressure testing methods.
Container cap manufacturing can involve processing using turret-based machinery. When a spark test unit is combined with such machinery, a spark test line or a separate spark test wheel is added to the equipment.
Spark testing requires a voltage that is proportional to the gap between the electrodes. To avoid having to use higher voltages, it is best to minimize the gap, to the point of contact between the electrodes and the cap. When doing so, high speed conveyance of the caps being tested becomes a challenge as the contact between the electrodes and the cap disrupts the flow.
It is known to perform a spark test using a star or pocket wheel fit with a spark test electrode associated with each cavity. The pocket wheel contains a plurality of cavities over which a plurality of mechanical vertical plungers bearing electrodes is installed. As the wheel rotates, the mechanical plungers insert the electrodes inside the caps being tested and the latter proceed to accomplish the high voltage spark testing. After the test is accomplished, the electrodes are lifted out of the caps. This system allows pressure to be exerted between the test electrode inserted into the cap cavity and the bottom of the cap. However this system requires a moving mechanism for each cavity of the pocket wheel. This requires additional floor space and increases the need for maintenance.
Some testing systems use a conveyor system that feeds caps to be tested with their interior side facing upwards with an electrode probe wheel having a number of probe “fingers” that rotate to move into and out of the cavities of the caps being conveyed. In these testing systems, the electrode probe wheel can turn as a result of contact with the conveyed caps or the electrode probe wheel can be driven by a motor turning independently of the conveyance of the caps. The tips of the probe fingers can be shaped to make good contact with the bottom of the caps.
German patent application publication DE102013014473 describes a belt conveyor with a vacuum unit in the middle and contains a spark testing unit. The testing unit takes the form of a rotating round-headed detecting device with probe fingers which comes in contact with vessels (i.e. caps) in order to apply a high voltage. An opposed stationary electrode is arranged between the vacuum belts, and the spark test thereby determines the presence of micro-holes. The vacuum belt conveyor provides the initial drive for the caps. The spark test probe wheel, whose probe fingers also makes contact with the inner sidewalls of the caps, sets a spacing between the caps fed by an input queue with the caps in contact with each other. The spark test is performed on each cap and output caps are spaced from each other by the vacuum belt. Providing a physical space between the caps to allow for a rejection of faulty vessels by the apparatus.
This approach can work well at high speed and provides a precise rejection of faulty caps or vessels. However, this approach does not allow for significant pressure to be exerted by the electrodes against the cap or container being tested, and thus requires a higher testing voltage.
Spark inspection may also be integrated into a “larger” automation solution for the manufacturing of plastic caps, such as slitting and folding of tamper evident band. In such a configuration, the current art requires that the spark testing be conducted on a conveying system that is outside of the overall equipment footprint.